No abstract

Most large molecules are chiral in their structure: they exist as two enantiomers, which are mirror images of each other. Whereas the rovibronic sublevels of two enantiomers are almost identical (neglecting a minuscular effect of the weak interaction), it turns out that the photoelectric effect is sensitive to the absolute configuration of the ionized enantiomer. Indeed, photoionization of randomly oriented enantiomers by left or right circularly polarized light results in a slightly different electron flux parallel or antiparallel with respect to the photon propagation direction-an effect termed photoelectron circular dichroism (PECD). Our comprehensive study demonstrates that the origin of PECD can be found in the molecular frame electron emission pattern connecting PECD to other fundamental photophysical effects such as the circular dichroism in angular distributions (CDAD). Accordingly, distinct spatial orientations of a chiral molecule enhance the PECD by a factor of about 10.

Photoionization is one of the fundamental light-matter interaction processes in which the absorption of a photon launches the escape of an electron. The time scale of this process poses many open questions. Experiments have found time delays in the attosecond (10−18 seconds) domain between electron ejection from different orbitals, from different electronic bands, or in different directions. Here, we demonstrate that, across a molecular orbital, the electron is not launched at the same time. Rather, the birth time depends on the travel time of the photon across the molecule, which is 247 zeptoseconds (1 zeptosecond = 10−21 seconds) for the average bond length of molecular hydrogen. Using an electron interferometric technique, we resolve this birth time delay between electron emission from the two centers of the hydrogen molecule.

We report on the non-adiabatic offset of the initial electron momentum distribution in the plane of polarization upon single ionization of argon by strong field tunneling and show how to experimentally control the degree of non-adiabaticity. Two-color counter-and co-rotating fields (390 and 780 nm) are compared to show that the non-adiabatic offset strongly depends on the temporal evolution of the laser electric field. We introduce a simple method for the direct access to the non-adiabatic offset using two-color counter-and co-rotating fields. Further, for a single-color circularly polarized field at 780 nm we show that the radius of the experimentally observed donut-like distribution increases for increasing momentum in the light propagation direction. Our observed initial momentum offsets are well reproduced by the strong-field approximation (SFA). A mechanistic picture is introduced that links the measured non-adiabatic offset to the magnetic quantum number of virtually populated intermediate states.

A central motivation for the development of x-ray free-electron lasers has been the prospect of timeresolved single-molecule imaging with atomic resolution. Here, we show that x-ray photoelectron diffraction-where a photoelectron emitted after x-ray absorption illuminates the molecular structure from within-can be used to image the increase of the internuclear distance during the x-ray-induced fragmentation of an O 2 molecule. By measuring the molecular-frame photoelectron emission patterns for a two-photon sequential K-shell ionization in coincidence with the fragment ions, and by sorting the data as a function of the measured kinetic energy release, we can resolve the elongation of the molecular bond by approximately 1.2 a.u. within the duration of the x-ray pulse. The experiment paves the road toward timeresolved pump-probe photoelectron diffraction imaging at high-repetition-rate x-ray free-electron lasers.

Following structural dynamics in real time is a fundamental goal towards a better understanding of chemical reactions. Recording snapshots of individual molecules with ultrashort exposure times is a key ingredient towards this goal, as atoms move on femtosecond (10−15 s) timescales. For condensed-phase samples, ultrafast, atomically resolved structure determination has been demonstrated using X-ray and electron diffraction. Pioneering experiments have also started addressing gaseous samples. However, they face the problem of low target densities, low scattering cross sections and random spatial orientation of the molecules. Therefore, obtaining images of entire, isolated molecules capturing all constituents, including hydrogen atoms, remains challenging. Here we demonstrate that intense femtosecond pulses from an X-ray free-electron laser trigger rapid and complete Coulomb explosions of 2-iodopyridine and 2-iodopyrazine molecules. We obtain intriguingly clear momentum images depicting ten or eleven atoms, including all the hydrogens, and thus overcome a so-far impregnable barrier for complete Coulomb explosion imaging—its limitation on molecules consisting of three to five atoms. In combination with state-of-the-art multi-coincidence techniques and elaborate theoretical modelling, this allows tracing ultrafast hydrogen emission and obtaining information on the result of intramolecular electron rearrangement. Our work represents an important step towards imaging femtosecond chemistry via Coulomb explosion.

We report on three-dimensional (3D) electron momentum distributions from single ionization of helium by a laser pulse consisting of two counterrotating circularly polarized fields (390 nm and 780 nm). A pronounced 3D low energy structure and sub-cycle interferences are observed experimentally and reproduced numerically using a trajectory based semi-classical simulation. The orientation of the low energy structure in the polarization plane is verified by numerical simulations solving the time dependent Schrödinger equation.

We report on a kinematically complete experiment on strong field double ionization of helium using laser pulses with a wavelength of 394 nm and intensities of 3.5 − 5.7 × 10 14 W/cm 2 . Our experiment reaches the most complete level of detail which previously has only been reached for single photon double ionization. We give an overview over the observables on many levels of integration, starting from the ratio of double to single ionization, the individual electron and ion momentum distributions over joint momentum and energy distributions to fully differential cross sections showing the correlated angular momentum distributions. Within the studied intensity range the ratio of double to single ionization changes from 2 × 10 −4 to 1.5 × 10 −3 . We find the momentum distributions of the He 2+ ions and the correlated two electron momentum distributions to vary substantially. Only at the highest intensity both electrons are emitted to the same direction while at the lowest intensity back-to-back emission dominates. The joint energy distribution of the electrons shows discrete structures from the energy quantization of the photon field which allows us to count the number of absorbed photons and thus access the parity of the final state. We find the energy of the individual electron to show a peak structure indicating a quantized sharing of the overall energy absorbed from the field. The joint angular momentum distributions of the two electrons show a highly directed emission of both electrons along the polarization axis as well as clear imprints of electron repulsion. They strongly change with the energy sharing between the electrons. The aspect of selection rules in double ionization which are also visible in the presented dataset has been subject to a preceding publication [1]. arXiv:1808.03516v1 [physics.atom-ph]